Because you haven’;t applied a texture with nipples, which most do in fact have, or because you haven’t bought or have bought but not used the body-shaping morphs for the figure.
You can fairly easily add nipples with DForms/magnets or in a modelling application and load as a morph, too. Anything even vaguely sexual has always been a paid for item - either in a non-free base figure or in a morph set for a free base (or, yes, a morph set for a non-free base).